1. Field of the Invention
The present invention generally concerns the disposal of waste, including hazardous waste, by the process of incineration. The present invention particularly concerns improvements to (i) the combustion of off-gas(es) produced by the process of pyrolysis, (ii) the shape and size of the burner within which off-gas(es) produced by the process of pyrolysis undergoes combustion, and (iii) the control of temperature at which pyrolysis transpires.
2. Description of the Prior Art
2.1 Classic Liquid-Injection Incineration
Most of the vast numbers of solid, liquid and occasionally gaseous material identified as hazardous waste are subject to thermal breakdown. Many types of waste, especially including petroleum wastes, also contain thermal energy, and are combustible.
Various forms of incinerators are most commonly used to handle liquid hazardous waste as well as hazardous wastes of other forms, i.e., solids sludges, slurries and fumes. In the field of hazardous waste incineration, there is more experience with liquid injection incinerators than all other types combined, with over half of all incinerators in use circa 1981 being of this type. In a liquid injection incinerator a pumped liquid is burned directly in a burner (combustor) or injected into the flame zone of the incinerator chamber (furnace) via nozzles. The heating value of the waste is the primary determining factor for nozzle location.
Liquid-injection incinerators are usually refractory-lined chambers in any of horizontal or vertical configurations, generally cylindrical, that are equipped with waste- and/or auxiliary-fuel-fired primary combustors and often also secondary combustors for low-calorific value materials such as aqueous wastes containing organic an/or inorganic compounds. The liquid-injection incinerators commonly operate in the approximate range from 1000.degree. C. to 1700.degree. C., with residence time in the chamber varying from milliseconds to a few seconds. With atomizing injectors high viscosity fluids of up to 4,500 SSU are capable of being incinerated.
All liquid-injection incinerators handling government-numerated hazardous waste compounds must meet the basic requirements of 40 U.S. Code of Federal Regulations Subpart 0-Incinerators. These regulations require: 99.99% destruction and removal efficiency (DRE) of the principal organic hazardous constituent (POHC); 99% removal of hydrogen chloride; and stack emissions not to exceed 180 mg. per dry standard cubic meter.
That this regulations can be met is proven by the large number of conforming liquid-injection incinerators in operation. However, these criteria are difficult to meet, especially for waste steams that vary in any of physical, chemical or thermodynamic properties.
The present invention will be seen to be flexible over a broad range, and at short times, to encompass significant variations in any of the physical, chemical or thermodynamic properties of the hazardous waste that is disposed of.
The burner(s) and/or combustion chambers of liquid-injection incinerators are normally quite large, on the order of cubic meters (m.sup.3), as are the incinerators themselves. This large size is predicated on the volume of hazardous waste stream for which it is economical to build an incinerator to dispose of. Even more intrinsically important, this large size is predicated on the thermodynamic requirement of sustaining a combustion, and normally a 1000+.degree. C. hot combustion, without any undue "quenching" of the combustion, and/or degradation of its efficiency, by contact of the combustion reaction with the generally cooler walls of the combustion burner, or chamber.
The present invention will be seen to function oppositely, achieving efficient and hot combustion in combustion burners, or "cups", that are relatively small, on the order of a few cubic centimeters (cm.sup.3).
The parameters, temperature, confinement, etc., of the incineration of mixed-waste streams, liquid or not, are often inherently contradictory. For maximum destruction and thermal breakdown of a principal organic hazardous constituent (POHC) the temperature is normally desirably high, and the dwell time long of the reaction long. Of course, a very long dwell time interferes with the incineration of fresh combustible material, and lowers the temperature of incineration due to the accumulation of the products of combustion in the incineration. For maximum destruction and thermal breakdown of a principal organic hazardous constituent (POHC) the incineration would normally best transpire in the presence of stochastic oxygen, and with turbulence so that the supplied oxygen is well reacted. One simple way to add oxygen to the incineration process is to supply abundant air to the process. But the air may cool the incineration temperature, diffuse the combustible material, and/or reduce the dwell time during which the incineration transpires. Finally, if the incineration is admirably hot for maximum destruction and thermal breakdown of the principal organic hazardous constituent (POHC), then the incineration may be so hot so as to cause miscellaneous materials and contaminants int he hazardous waste steam, primarily metals, to be undesirably taken up into the gases of the incineration, and emitted.
2.1 Classic Pyrolysis
Pyrolysis, sometimes referred to as thermal decomposition or destructive distillation, is defined as the destructive distillation of a stream of carbonaceous material in the presence of heat and in the absence of stoichiometric oxygen to produce (i) an off-gas with a combustible content and (ii) char. Application of pyrolysis to the treatment of waste, including hazardous waste, leads to a two-step disposal process. In the first step, the wastes are heated, normally in the complete absence of oxygen, in order to separate volatile components of combustible gases, water vapor, etc., from non-volatile char and ash. In the second step, (i) the volatile components are burned in the presence of a gaseous mixture containing oxygen in order to incinerate all noxious and/or hazardous components as best and as completely as is possible, while (ii) the solid char and ash is disposed of as solid waste.
Several variations of waste treatment by pyrolysis include (i) energy recovery and/or (ii) recovery of solid components from the volatile components an/or solid residual.
High-efficiency low-emission reduction of hazardous waste by the two-step processes of (i) pyrolysis and (ii) burning presents at lest two challenges. First, the heat of the pyrolysis reaction is desirably kept very nearly constant at a level that establishes the proper and desired balance between the solid char and off-gas(es) produced, and that never becomes so hot so as to, for example, force metals into the stream of off-gas(es). However, if the waste stream varies in its thermal component and/or thermal absorption, both of which are common, then the heat of pyrolysis is difficult to control.
Second, the solid char contains some combustion energy, even if it is not desired to liberate this energy by burning because of the (typically considerable) negative consequences, typically pollution emission, attendant upon any burning of the char. (The char is normally buried, or incorporated in the matrix of another material such as asphalt.) The combustion energy of the off-gas(es) is thus equal to the combustion energy of the original waste stream diminished by the combustion energy remaining in the char. The combustion energy of the off-gases is usually considerable, but no more so than is needed to establish a combustion temperature when burning the off-ga(es) that is optimal for reduction of noxious and/or odorous components within the off-gases (and/or the off-gases themselves). Accordingly, when pyrolysis is conducted than the disposal of hazardous waste has only begun, and much attention would desirably also be paid to the subsequent combustion of the off-gases produced by the classical process of pyrolysis.